A writing instrument having a tubular writing tip connected to an end of a barrel or casing is known, in which ink is fed from an ink reservoir in the barrel to the tubular writing tip. In such a writing instrument, an axially slidable needle or rod is usually provided within the tubular writing tip so as to improve ink feeding.
The writing instrument of the type as described has been used primarily by draftsmen. For drafting, synthetic resin films treated to have a rough surface, such as by sandblasting and chemical mats, have been used, which, however, cause remarkable wearing of the tubular writing tip. In order to minimize such wear, an attempt has been made in which an additional tubular nib made of a hard materials having high wear-resistant properties such as a metal, for example hard stainless steel, chrome-plated brass and the like, precious stones such as ruby and the like, or ceramics, is fitted to an end of the aforementioned tubular writing tip.
The tubular nib is conventionally fitted to the tubular writing tip by employing desired adhesive agents or by press-fitting. During connection by the adhesive agents, the tubular nib is formed with its outer diameter being smaller than an inner diameter of the tubular writing tip so that an annular space is formed between the nib and the tip, and the adhesive agent is supplied into the space. During the press-fitting connection, the tubular nib is formed with its outer diameter being substantially equal to, or slightly larger than, the inner diameter of the tubular tip, and the nib is forcibly inserted into the tip.
In the connection by the adhesive agent, however, it is quite difficult to hold both the annular nib and the annular tip in accurate positions with their axes being aligned with each other since the annular space or gap between the nib and the tip must be formed for inserting therein the adhesive agent. When both the annular nib and the tip are not accurately positioned, thus resulting in non-alignment of their axes, a constant flow of ink will not be expected, and a smooth axial movement of a needle or rod will not be established when the writing instrument is of the type which employs such needle or rod in the tubular tip. In the press-fitting connection, finishing of highest precision is required for the nib and tip since the outer diameter of the nib and the inner diameter of the tip must be controlled with strict dimensional accuracy. To be more specific, an outer diameter of the nib larger than the predetermined dimension will not permit the nib to be inserted into the tip and, on the other hard, a smaller diameter of the nib will apparently fail to firmly connect the two elements together. Besides, even though both the nib and the tip could be formed in a desired dimensional accuracy, problems are still unsolved in that the more accurately are the elements formed, the more accurate must be the press-fitting operation. If a deviation, however small it may be, is produced between the nib-advancing direction and the axial direction of the nib, an excessive thrust is needed to press the nib further into the tip. Such excessive thrust will cause the two elements to be deformed or damaged.
The aforesaid problems encountered during the connection of the elements are due to the fact that both the nib and the tip are extremely small sized, and that even the smallest deviations in the press-fitting direction and in dimensional accuracy result in a larger disadvantageous effect.